To determine which complexes show optical activity, we need to consider the presence of chiral centers and symmetry elements.

I = [Ru(bipyridyl)3]
II = [Cr(EDTA)]-
III = trans[Cr(Cl)2(oxalate)2]3-
IV = cis[Cr(Cl)2(oxalate)2]3-

I. [Ru(bipyridyl)3]
This complex contains a central metal ion (Ru) bonded to three bipyridyl ligands. The bipyridyl ligands are not chiral, and the complex does not possess any chiral centers. Therefore, complex I does not show optical activity.

II. [Cr(EDTA)]-
This complex contains a central metal ion (Cr) bonded to the EDTA ligand. The EDTA ligand is chelating and forms a six-membered ring with the metal ion. The complex does not possess any chiral centers, and there are no symmetry elements that can lead to optical activity. Therefore, complex II does not show optical activity.

III. trans[Cr(Cl)2(oxalate)2]3-
This complex contains a central metal ion (Cr) bonded to two chloride ions and two oxalate ligands. The oxalate ligand is chelating and forms a six-membered ring with the metal ion. The complex possesses a chiral center due to the presence of the oxalate ligands. However, the trans configuration suggests that the two chiral centers cancel each other out, resulting in an overall achiral complex. Therefore, complex III does not show optical activity.

IV. cis[Cr(Cl)2(oxalate)2]3-
This complex contains a central metal ion (Cr) bonded to two chloride ions and two oxalate ligands. The oxalate ligand is chelating and forms a six-membered ring with the metal ion. The complex possesses a chiral center due to the presence of the oxalate ligands. The cis configuration suggests that the two chiral centers do not cancel each other out, resulting in an overall chiral complex. Therefore, complex IV shows optical activity.

In summary, the complexes that show optical activity are:

a) I
b) II
d) IV